(i) Field of the Invention
The present invention pertains in one aspect to an improved version of a dry thermal processor for extracting volatile substances from a particulate host material. The processor is of the type incorporating horizontal, concentric, substantially co-extensive, inner and outer tubular members which are interconnected and which rotate together about a horizontal axis. The feedstock enters at one end of the inner tubular member, advances through it, and is heated by hot solids returning through the annular space between the tubes.
In another aspect, the invention pertains to an improved version of the process wherein the feedstock is initially advanced through the inner tubular member and is heated in two stages, firstly to vaporize water contained in the feedstock and secondly to pyrolyse hydrocarbons and produce coked solids. The coked solids are transferred into the annular space, wherein the coke is burned to produce hot solids. Part of the hot solids is recycled into the hydrocarbon vaporization or reaction zone to provide needed heat for that zone. The balance of the hot solids is returned through the annular space and is used to transfer heat into the water vaporization or pre-heat zone by contact with the wall of the inner tubular member.
(ii) Prior Art
The present invention relates to improved versions of the processor and the process disclosed in U.S. Pat. Nos. 4,280,879 and 4,285,773.
A pilot plant-scale processor in accordance with the patents was built and operated on an experimental basis for a number of years. In the course of the work, certain problems were ascertained and solutions to the problems were developed. The processor and its method of operation were significantly modified. The modified versions of apparatus and process provide the basis for the present invention.
The patented processor was originally designed with the primary objective of extracting hydrocarbons from the oil sands of the Athabasca region in Northern Alberta. Such oil sands typically comprise grains of sand individually sheathed in a thin membrane of connate water. The water contains minute clay particles. Bitumen is trapped in the interstices between the water-sheathed sand grains. Stated otherwise, oil sand is a mixture of particulate solids, water and hydrocarbons. The prior processor was designed to recover some of the hydrocarbons, separate from the water and solids.
In the course of the piloting work, the patented processor and its method of operation were shown to be applicable to feedstock other than oil sand. Such feedstock also involved a mixture of particulate solids, water and volatile substances (including hydrocarbons). More specifically, the processor was operated to treat crushed oil shale and contaminated soil mixtures from waste dumps, with beneficial results.
In its original form, the patented processor broadly involved the following:
A pair of concentric, substantially co-extensive, horizontal, radially spaced apart inner and outer tubes (sometimes referred to as "tubular members") were provided. the tubes were rigidly interconnected and adapted to be rotated together about their longitudinal axis;
There was thus formed an enclosed, elongate, cylindrical inner space and an enclosed outer annular space. These spaces or passageways were "open", in the sense that they were substantially unobstructed except as described below;
The cylindrical inner passageway was divided at a point along its length by a transverse baffle into an upstream water vaporization zone (or "pre-heat" zone) and a downstream hydrocarbon vaporization zone (or "reaction" zone). The baffle was supplied to assist in segregating the gaseous atmospheres of the pre-heat and reaction zones. Spiral open-ended chutes were associated with the baffle and formed passages extending through the baffle at its periphery. These passages enabled solids to move from the pre-heat zone into the reaction zone. The presence of the solids in the chutes combined with the presence of the baffle itself to substantially prevent the movement of gases from one zone to the other;
A conveyor extended through a first end frame for feeding feedstock into the first end of the pre-heat zone;
Screwing elements, such as upstanding plates angled relative to the longitudinal axis of the inner tube, were secured to the inner surfaces of the inner and outer tubes, to add fine control for advancing or retarding the movement of solids through the inner space and the annular space;
A first fan system, having a conduit extending into the pre-heat zone, provided suction and means for withdrawing water vapour and light hydrocarbon vapours from said zone;
A second fan system, having a conduit extending into the reaction zone, provided suction and means for withdrawing hydrocarbon vapours therefrom;
A baffle and seal assembly was provided at the second end of the inner tube. This baffle and seal assembly was also of the previously described spiral chute type and was adapted to prevent gas movement between the reaction zone and the annular space, while still enabling coked solids to move from the reaction zone into the second end of the annular space;
The annular space provided a combustion zone at its second end and a heat transfer zone at its first end;
An air injection system was provided to supply pre-heated air through the second end frame into the combustion zone, for supporting combustion of the coked solids;
A gas burner fire tube also projected through the second end frame into the combustion zone;
A recycle assembly, connecting the annular space with the first or upstream end of the reaction zone, was provided at the first or downstream end of the combustion zone, for transferring some of the hot solids, leaving the combustion zone, back into the reaction zone. The recycle assembly involved a spiral chute coiled around the inner tube and extending through the tube wall. The chute was adapted to scoop hot solids from the annular space and, as a result of rotation with the inner tube, to deliver the solids to the combustion zone. The chute and its load combined to substantially prevent gas movement between the annular space and the reaction zone;
There were lifter elements attached to the inner surface of the outer tube in both the combustion and heat transfer zones. In the combustion zone, these lifters would drop the coked solids particles in dispersed, curtain-like fashion through the injected air, to encourage combustion. In the heat transfer zone, the hot solids were lifted and cascaded onto the pre-heat portion of the inner tube, to supply heat to the tube wall by solid-to-solid heat transfer;
A third fan system, having a conduit extending into the annular space, provided suction and means for withdrawing the flue gases therefrom; and
Means, such as a conveyor, extended through the first end frame for removing cooled solids from the downstream end of the annular space.
In the operation of the prior art processor, the following occurred:
The feedstock was heated in the pre-heat zone by heat transfer through the tube wall. In the case of oil sand, large cohesive chunks were ablated by the heating and mild cascading action within the rotating inner tube. Contained water and the lightest, low boiling point hydrocarbons were vaporized and removed by the first fan system. And the contained rocks were freed from the rest of the oil sands so that they could be separated by screening at the downstream end of the zone and removed from the main feed stream;
In the reaction zone, the pre-heated feed was mixed with hot solids recycled from the annulus, to thereby raise the temperature of the feed. Hydrocarbons were vaporized and cracked. Residue coke formed on the solids particles. And the hydrocarbon gases were separately recovered by the second fan system;
In the combustion zone, the coked solids were lifted and dropped through the injected air and burned to yield hot solids. The solids were also heated in part by the auxiliary heater. Part of the hot solids was recycled into the reaction zone, to supply the heat needed to raise the temperature of the feed to the desired hydrocarbon vaporizing/cracking temperature. And the balance of the hot solids was advanced into the heat transfer zone of the annulus;
In the heat transfer zone, the hot solids were lifted and dropped onto the pre-heat portion of the inner tube, to heat the inner tube wall as required;
And the suction systems plus the seal devices were used to substantially isolate the pre-heat, reaction and annular zone gaseous atmospheres, one from another.
In a broad context, the processor can be characterized as a self-powered heat transfer machine. Among the factors that require attention in its design are the following:
Heat must be transferred from the hot solids, moving through the annular space, to the cool solids moving through the pre-heat zone. The transfer of heat must be sufficient so that the exit temperature of the bed of feed in the pre-heat zone is raised from ambient to a temperature at which vaporization of water contained in the feed will be essentially complete, without significant vaporization of hydrocarbons. In the case of oil sand, this exit temperature should typically be about 550.degree. F.;
Such transfer of heat is affected by the extent of contact between the hot solids and the pre-heat zone tube wall, the temperature and volume of the hot sand cascaded, the conductance of heat through the tube wall, the transfer of heat from the tube wall into the feed bed, and the movement of heat through the bed itself;
Combustion of the coked solids and auxiliary fuels must be sufficient to raise the temperature of the solids to the desired value (in the case of oil sand, typically about 1300.degree. F.), needed to satisfy the heat demands of the pre-heat and reaction zones;
The quantum of heat transferred into the reaction zone by recycle of hot solids must be sufficient to achieve the increase of temperature of the feed in the reaction zone which is needed to crack the hydrocarbons and produce lighter molecular weight hydrocarbons and coked solids;
The foregoing factors must be obtained while maintaining segregation of the gaseous products, so that contamination and hydrocarbon losses are minimal; and
The machine is subject to elongation, expansion and contraction due to variations in temperature to which it is subjected. The outer tube is internally insulated and thus is not heated to a high temperature. The inner tube is, however, heated to high temperature. Therefore, there is a significant difference in the axial and radial expansions of the two tubes. Therefore, the processor needs to be designed to accommodate the relatively different physical changes which occur with heating.
It will be understood that there are a number of operating parameters which become generally fixed. For example, the rate of feed addition, the rate of recycle of hot sand to the reaction zone, and the rate of hot sand movement through the annular space all become relatively steady.
It also will be understood that, for the majority of operations, addition of supplemental heat is to be minimized, as auxiliary or off-site fuel would be a significant cost factor in the operation of the processor.
And it will further be understood that the machine should be kept as short as possible.
With the foregoing background in mind, it is now appropriate to summarize the invention.